The present invention relates to an inverter apparatus, and in particular, to an inverter apparatus having a restarting function at an instantaneous power failure which is constituted so as to decrease the peak value of a restarting current as much as possible by applying a voltage conforming to a phase of a residual voltage of an induction motor (IM) connected to an output of the inverter apparatus at the recovery of the instantaneous power failure.
FIG. 1 shows a circuit diagram of an inverter apparatus having a conventional restarting function at an instantaneous power failure, including a converter portion for rectifying an AC power to a DC power; a smoothing capacitor 2 (CB) for smoothing a rectified DC power in the converter portion and an inverter portion 3 for converting DC power to an AC power having an arbitrary frequency. There is also provided a current detector 4 for detecting an output current of the inverter portion, an induction motor 5 connected to the output of the inverter apparatus, a phase detector 6, a power failure detector circuit 9 and an inverter control circuit 10. In the inverter apparatus, terminals R, S and T are connected to a power supply, and the AC power from the power supply is converted to an AC power having an output frequency corresponding to an instructed frequency, (not shown) and supplied to the induction motor 5 connected to the output thereof.
A conventional restarting operation at the occurrence of an instantaneous power failure in a power supply when the inverter apparatus of FIG. 1 is operating the induction motor at a variable speed will be explained in the following.
When a power failure occurs, after the lapse of a guaranteed period of time for the continuation of operation at an instantaneous power failure, base currents of transistors QR, QS, QT, QU, QV and QW, provided in the inverter portion 3 are cut off, and the motor runs in a free-running state. A secondary current which flows in a rotor of the induction motor 5 decreases according to an electric time constant, so that generally the residual magnetic flux remains for 0.1 to 1 sec. A residual voltage is generated in a primary side by the residual magnetic flux. The restarting of an operation at an instantaneous power failure means an operating method for restarting an induction motor by re-applying a voltage thereto when the power is recovered while the residual voltage is still being generated. In this case, if the re-applied voltage is opposite in phase to the residual voltage, an over current flows and an over current trip can be operated. In a conventional case, the phase detector 6 is provided on the output side of the inverter apparatus for detecting the residual voltage of the induction motor, and the inverter is controlled to output a voltage in phase with the detected residual voltage.
A circuit using a transformer for voltage detection (PT) or a photo-coupler 7 (PHC) as shown in FIG. 2 has been used as the phase detector 6 in order to separate a high voltage portion in a main circuit from a low voltage portion in a control circuit. In a conventional system, a transformer for voltage detection, which is not generally used in an inverter apparatus for general usage, is utilized, and there has been a problem such that the transformer increases the price of the inverter apparatus or prevents miniaturizing of the inverter apparatus. In the case of the phase detector 6 using the photo-coupler 7 (PHC), it is necessary to limit a current which flows in the photo-coupler, PHC, to be about 15 mA, so that, assuming that an effective voltage in the high voltage portion is 220 V, a current limiting resistor 8 (RUV) requires a capacity of 3.3 W, 15 k.OMEGA., and a resistor of about 10 W will be used in consideration of reliability and allowance for heat capacity. Because of this, the size of the resistor itself becomes large, and there has been a problem of difficulty in miniaturizing the inverter apparatus in consideration of heat radiation. There has been also a problem that reliability may be degraded by the increase in the number of parts of the inverter apparatus.